1. Field of the Invention
The present invention relates to a controller for internal combustion engine and a vehicle mounting the controller. More specifically, the present invention relates to control of idle speed setting of an internal combustion engine.
2. Description of the Background Art
In an internal combustion engine, engine speed in a self-sustained operation with driving power not transmitted to a load after engine start, that is, the engine speed in idle operation or idling (hereinafter also referred to as “idle speed”) should desirably be as low as possible while allowing self-sustained operation, in order to reduce fuel consumption.
While the engine is in operation, engine operation causes vibration. In order to reduce vibration during idle operation, the idle speed is set to be higher than the speed that causes resonance in driving power transmission system including the engine (hereinafter also referred to as “resonant speed”).
Japanese Patent Laying-Open No. 2006-152877 (Patent Document 1) discloses, in a hybrid vehicle having a mounted engine cranked by a starter motor, a configuration in which, if there is a possibility that engine speed at the time of cranking becomes equal to the resonant speed of driving power transmission system as the increase of engine speed is curbed during engine cranking, the motor is driven to attain engine speed lower than the resonant speed.
According to the configuration disclosed in Japanese Patent Laying-Open No. 2006-152877 (Patent Document 1), even if there is a possibility that the engine speed becomes equal to the resonant speed because of decrease in motor output resulting from increased friction torque or decrease of battery output at the time of cranking for engine start operation, resonance in the driving power transmission system can be prevented.
Generally, the engine idle speed is set to a value different from the speed (resonant speed) corresponding to the resonant frequency of the driving power transmission system to which vibration from the engine propagates, in order to reduce vibration during idle operation.
It is noted, however, that the resonant speed of driving power transmission system changes if the vehicle is kept in a low-temperature environment (for example, lower than −15° C.) with the engine stopped, for example, in cold climates. Therefore, if the vehicle is kept in a low-temperature environment with its engine stopped, the resonant speed of driving power transmission system comes closer to the idle speed, possibly increasing vibration in the idle operation.